Interferons are proteins that are released by host cells in response to the presence of a virus. They play a crucial role in the immune system`s defense against viral infections. When a virus enters the body, infected cells secrete interferons as a signal to neighboring cells. This signaling helps to prepare nearby cells to defend against the virus.

Options 1, 2, and 3 (Bacteria, Fungi, and Mycoplasma) are incorrect because interferons are not synthesized in response to these entities. Interferons primarily target viruses, although they can have some effect on other pathogens. It`s important to note that interferons are not effective against all viruses and their efficacy can vary depending on the specific viral infection. Therefore, the correct answer is option 4: Virus.

In conclusion, interferons are synthesized in response to viral infections as a defense mechanism of the immune system.